Boxwood suffers from new fungal disease

Q: I heard there is a new problem with boxwood. Is this true? — Roger from Central Valley

Debbie Lester

Q: I heard there is a new problem with boxwood. Is this true? — Roger from Central Valley

A: There is a new fungal disease affecting boxwood. The problem, called "box blight" or "boxwood blight," was reported from both Connecticut and North Carolina in October 2011.

Since its initial U.S. detections, the problem has also been identified in New York, Pennsylvania and several other states, as well as three Canadian provinces.

The pathogen has apparently been moved by shipping diseased plants; other states where boxwood is grown will very likely be discovering it shortly.

Boxwood blight is caused by a fungus that was only recently identified — Cylindrocladium pseudonaviculatum, also known as C buxicola and Calonectria pseudonaviculata. Prior to its appearance in the United States, it was observed for the very first time in the United Kingdom in 1994.

Initially, symptoms are round, black or dark brown leaf spots and thin, black streaklike shoot cankers, either of which may be easily overlooked.

Under warm, humid conditions, the disease quickly progresses to show large areas of browned leaves and conspicuous leaf drop.

In a diagnostic laboratory equipped with microscopes, the leaves and twigs can be examined for the characteristic fruiting of the fungus. It is important to distinguish this new Cylindrocladium blight from other common boxwood problems such as Volutella blight and Macrophoma blight — it is impossible to tell which disease is affecting boxwood by field observations alone.

The boxwood blight pathogen is comfortable in temperature conditions typical in spring and fall in the northeastern U.S., but it does not do well at high summer temperatures. It can grow below 50 degrees F, but growth is halted at 86 degrees and the fungus mycelium is killed at 95 degrees (although microsclerotia within killed leaves remain alive).

The disease is most favored by warm (64-77 degrees), humid conditions. A complete disease cycle (from infection to sporulation) can take as little as seven days.

In 2012 on Long Island, boxwood blight dramatically affected plants in both Nassau and Suffolk county landscapes just after a very humid period in early September.

Host susceptibility within boxwood in tests in the UK ranged from the leathery-leaved B. balearica as the most resistant, to B. sempervirens Suffruticosa as the most susceptible.

This new species of Cylindrocladium will be very difficult to control with fungicides. Masking the problem with fungicides rather than taking more strict eradication actions would be a mistake — the disease will then become extensively distributed as a result. Virginia researchers have observed microsclerotia forming in fallen leaves, and this means that the debris from diseased plants is very dangerous.

Sanitation efforts after a disease outbreak must be extremely thorough. Infected branches should be pruned out in dry weather, and fallen dead leaves should be collected and bagged. Because the pathogen can last in dead leaves for at least five years, composting is not a good option.

Debbie Lester is the community horticulture educator at the Cornell Cooperative Extension of Orange County.

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